Cathodic protection is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. This method is often used to protect metal structures from corrosion. Cathodic protection systems are commonly used to protect steel, water, fuel pipelines, tanks, steel pier piles, ships, and offshore oil platforms. An undesirable side effect of improperly performed cathodic protection is the generation of molecular hydrogen, leading to its absorption in the protected metal and subsequent hydrogen embrittlement of said metal.
Historically water storage tanks have had cathodic protection systems designed to operate over a large range of loads. In years past, a typical 3 million gallon storage tank with a new coal tar coating would require a small amount of cathodic protection current. However the old technology coal tar coatings would degrade in time. As the coating degrades the cathodic protection current requirement increases. As a result of the expected coating degradation, most cathodic protection systems installed were Impressed Current Cathodic Protection (ICCP) systems which use anodes connected to a DC power source (a cathodic protection rectifier) to provide the necessary current levels. The operating output of the rectifier is adjusted to an optimum level by an operator after conducting various voltage measurements of the tank to water potentials.
In recent years, newly developed high tech epoxy coatings have proven to have long term durability and stable dielectric efficiency. As a result a typical 3 million gallon reservoir with a new epoxy coating requires relatively little current (e.g., less than 100 milliamperes of current), and can easily be protected by a galvanic cathodic protection system utilizing sacrificial anodes. Galvanic anodes for cathodic protection are typically made from various alloys of magnesium zinc and/or aluminum. The electrochemical potential, current capacity, and consumption rate of these alloys are well suited for cathodic protection. Galvanic anodes are designed and selected to have a more “active” voltage (technically a more negative electrochemical potential) than the metal of the structure being protected (e.g., tank, etc.). For effective cathodic protection, the potential of the structure is polarized more negative until the corrosion reaction is halted. The galvanic anode continues to corrode, slowly consuming the anode material. The difference in electrochemical potential between the anode and the cathode causes current to flow from the anode to the structure (cathode).
The American Water Works Association (AWWA) standards and National Association of Corrosion Engineers (NACE) recommended practices suggest that tank-to-water potentials be maintained between −0.850 and −1.100 volts with respect to a stable copper-copper sulfate reference electrode. Exceeding the −1.200 volt potential limit has demonstrated in some instances to be detrimental to the coating by “over protection” which may produce hydrogen and cause the protective coating in the tank to degrade, lift, separate from the tank wall. Therefore, when using a magnesium anode there is a need to provide some method to prevent over potentials. A resistor system may be installed in series with the magnesium anodes to limit the current output. However, this type of control system requires frequent adjustments since the voltage potential in a tank may change as the amount of liquid held in the tank changes. Another option is to install an automatic-constant potential ICCP system.
It is common practice for cathodic protection designers to specify that the tank to water potential be an “IR Free” measurement. That is, the measurements are performed while the no current flows between the anode and tank (e.g., at an instant when cathodic protection is turned off). This is not a problem with the modern IR free impressed current rectifier systems. However, because most magnesium anode systems are “On” continuously, it is very difficult/impractical to capture a true IR free potential measurement.
With the improved epoxy coating systems, it is not unusual to provide full protection for a 3 million gallon reservoir with less than 100 milliamperes of current. This has created a double-edged sword for cathodic protection designers. With the low current requirement, a newly coated tank is a perfect candidate for a magnesium system. However, even the lowest rated current output cathodic protection rectifiers have difficulty operating in the milliampere range. They tend to be unstable and difficult to adjust at the low end of this operating power range. The current fix for this problem is to install a “dummy” load or balancing resistor across the output to “fool” the rectifier into believing that it is operating at a higher current.
A corrosion protection system using a magnesium anode would require series balancing resistor to keep the anode-to-tank current low enough to avoid over protection. However, using such balancing resistor requires frequent adjustment to maintain the desired potential range while the problem of capturing IR free potentials still persists. The need for adjustment may occur for various reasons such as, for example, a change in the water level of the tank. Thus, there is a need for an IR free automatic potential controlled impressed current system.
Additionally, providing power to operate the circuitry of an automatic potential control cathodic protection system is very difficult in many cases. For example, water storage tanks are often located in remote areas where an independent power source to operate the potential control circuits is unavailable. Thus, a way or method to provide power to such automatic control system is needed.